1. Field of the Invention
The invention relates to integrated circuits for operating liquid crystal displays, and more particularly to a liquid crystal display driver using a non-complementary metal oxide semiconductor (MOS) integrated circuit structure.
2. Description of the Prior Art
It is known that to obtain long life, a high contrast ratio, and minimal "ghosting", a liquid crystal display (LCD) must be driven with a carefully characterized ac voltage. In particular, the ac voltage should assume a substantially zero value for non-illuminated portions of the display. The presence of a small ac or dc voltage, for instance in excess of 0.05 volts, may cause the appearance of "ghosts"--or dimly activated segments in the non-activated portions of the display. In addition, if in the operation of the segments, a dc component persists on the segments, there will be a gradual loss of clarity in the display. These requirements are somewhat more severe for liquid crystal displays utilizing a colored dye dichroic material to which the present drivers have application. The dichroic liquid crystal displays have somewhat lower thresholds and require higher RMS drive voltages.
In general LCD driving circuits, where the cost of the IC need not be minimized, it is conventional for the LCD driver to be constructed with a Complementary Metal Oxide Semiconductor (CMOS) Field Effect Transistor (FET) process.
In a complementary driver PMOS and NMOS devices may be used to drive each side of the display to the plus and minus power supplies respectively on alternating cycles so that there is a negligible dc component in the ac activation provided to the active segments of the display. In short, the performance of the complementary MOS drivers can be very good, but at a cost which is substantially greater than that of a non-complementary (NMOS or PMOS) integrated circuit.
Known non-complementary integrated driving circuitry for LCD displays has often required compensatory features to be functional, such as discrete backplane capacitors to block dc. Often oversized drivers for each of the segments have been required resulting in both excessive chip areas and excessive power demands. A recurrent problem with non-complementary display drivers has been the appearance of "ghosts" in off segments and the presence of some average level of dc which has tended to reduce the clarity of the display. The present invention seeks to provide non-complementary (PMOS or NMOS) drive circuitry, which avoids the problems of past non-complementary LCD drive circuitry, and has a performance comparable to that of complementary (CMOS) drive circuitry.